This invention relates to electronic inverter circuits and more particularly to such circuits which are capable of supplying a constant voltage-to-frequency ratio output.
In airborne electrical power generation systems, it is desirable to have a single system which provides both the starter and generator functions. The weight savings on an airplane can be substantial when a dedicated starter is eliminated. For this reason, electrical power systems which are capable of providing engine start functions can provide both cost and weight savings.
Variable speed constant frequency (VSCF) power generation systems are commonly used for aircraft applications. One type of VSCF system includes a variable speed generator which supplies DC power to a pair of DC link conductors. An inverter circuit receives DC power from the link conductors and produces a constant frequency AC output. The inherent simplicity and reliability of DC link VSCF systems has been established and it is desired to modify the existing designs to provide starter capability.
DC link inverters commonly utilize a transistor bridge output circuit having a pair of series connected transistors in each output phase leg adapted to be connected between the DC link conductors, wherein a connection point between the transistors serves as an output power pole. The transistor drive circuits may include a controlled current feedback transformer (CCFT) which uses the transistor's collector current, through the CCFT turns ratio, to provide the required base drive in a self-supporting manner. The CCFT provides dielectric isolation between the power output transistors and the control circuits. It is necessary to keep the CCFT from saturating, otherwise loss of drive and loss of control of the inverter will result. U.S. Pat. No. 3,715,648, issued Feb. 6, 1973 to Kernick et al. discloses an inverter which utilizes a CCFT in the output transistor drive circuit and is hereby incorporated by reference.
One method of converting a DC link VSCF system to a starter/generator system is to use a contactor or set of contactors to turn the system around so that an external AC source supplies the inverter input voltage and the generator is connected to the inverter as a load. If the generator, acting as a motor in this case, is a synchronous machine, it will be necessary to provide AC current from the inverter at a frequency near zero hertz. Generally this has previously been considered impractical in inverters employing CCFT drive circuits since any frequency much below the normal rated frequency, for example 400 hertz, will result in saturation of the CCFTs in the transistor drive circuits.